The present invention relates to a micromachining method employing a focused ion beam and particularly to such a method for micromachining solid materials, for example on semiconductor devices, in a selective and environmentally acceptable manner.
Focused ion beam systems are used in forming, shaping or altering microscopic structures in semiconductor devices, or in other solid materials, including semiconductor or insulating surfaces and metal areas providing electrical connection. The focused ion beam can be directed to a very small point on a semiconductor device and then scanned, raster fashion, over a surface where material is to be removed. As an ion impinges on the semiconductor device surface, its momentum is transferred resulting in the removal of one or more surface atoms according to a process called sputtering. By selecting a raster pattern of a given overall shape, for example a horizontal raster pattern, a correspondingly shaped area of surface material can be removed. Often several successive layers of a semiconductor device are removed in a given area in order to reach and possibly sever an underlying layer.
It has been discovered heretofore that the above described physical sputtering process can be enhanced by the introduction of a chemically reactive gas, i.e., fluorine or chlorine. The gas is adsorbed on the surface of the semiconductor device prior to arrival of the ion beam and the gas reacts chemically with the surface material to produce an area of material that is more easily sputtered away. Thus a gas such as chlorine chemically reacts with the surface and forms chlorides which are more easily removed by the ion beam, resulting in a substantial increase in sputtering rates. For instance, an enhancement factor in sputtering of silicon has been reported of approximately fourteen, i.e., the gaseous enhanced sputtering occurs approximately fourteen times as fast as without the introduction of the gas. Gas enhanced sputtering is also less subject to redeposition of sputtered material. The enhancement factor for metal surfaces such as conductive layers in the semiconductor device may be even greater.
However, the introduction of chlorine or fluorine gas creates a safety hazard because of the combined toxicity and high vapor pressure of the gas. Consequently the use thereof is outlawed in many industrial settings. Moreover, handling of tanks of toxic gas at high pressures is awkward even in a carefully monitored laboratory environment.
Furthermore it is difficult to confine the area which is to be machined only to the area upon which the focused ion beam impinges. As indicated above, chlorine gas, for example, is often chemically reactive with the whole surface of the object being formed and may undesirably attack areas adjacent to the ion beam position resulting in a lack of contrast between machined and non-machined areas.